A 6-axis manipulator, illustrated in FIG. 23, as one type of an articulated robot includes a first articulated drive system, which includes a J4 axis, a J5 axis, and a J6 axis for changing an attitude of an end effector (working part) disposed at a forward end, and a second articulated drive system, which includes a J1 axis, a J2 axis, and a J5 axis for changing a position of the first articulated drive system. A control device for the 6-axis manipulator changes the position and the attitude of the end effector by controlling the J1 axis to J6 axis based on respective positions and attitudes of the end effector at a work start point and a work end point that are set in advance.
In more detail, plural interpolation points for interpolating a teaching path interconnecting the work start point and the work end point are calculated, and the J1 axis to the J6 axis are driven in accordance with the calculated interpolation points. When position coordinates X, Y and Z and attitude angles α, β and γ of the end effector are given, respective articulation angles θ1 to θ6 of the J1 axis to the J6 axis can be determined by finding the solution of an inverse kinematic problem. However, when the inverse kinematic problem is solved, there are two kinds of solutions for the articulation angles θ1 to θ6. More specifically, the angles θ′4 and θ′4 of the J4 axis, the angles θ5 and θ′5 of the J5 axis, and the angles θ6 and θ′6 of the J6 axis in the two kinds of solutions are related to each other by expressions of θ4−θ′4=±180 degrees, θ5+θ′5=0 degree, and θ6−θ′6=±180 degrees.
Meanwhile, when the angle of the J5 axis becomes 0 degree in the 6-axis manipulator, the manipulator takes the so-called singular attitude (singular point) at which speeds of the J4 axis and the J6 axis change abruptly. In general, therefore, the singular attitude is avoided by causing the angle of the J5 axis to change in the same sign as that of the solution at the work start point. In that case, however, when the angle of the J5 axis takes different sings signs at the work start point and the work end point, the angle of the J5 axis cannot be made to reach an angle to be taken at the work end point.
On the other hand, Patent Literature 1, for example, discloses a technique for transiting the angle to the other solution of the inverse kinematic problem when it is detected that the manipulator is in the vicinity of the singular attitude. With such a technique, even when the angle of the J5 axis takes different signs at the work start point and the work end point, the angle of the J5 axis can be made to reach the angle to be taken at the work end point. In general, the singular attitude is detected on condition that the angle of the J5 axis reaches the vicinity of 0 degrees, or that the angles of the J4 axis and the J6 axis change abruptly.